1. Field of the Invention
This invention relates to apparatus used in downhole cementing of well casing, and more particularly, to a stage cementer with an integral packer and an improved means for retaining internal seats therein.
2. Description of the Prior Art
In preparing oil well boreholes for oil and/or gas production, a most important step involves the process of cementing. Basically, oil well cementing is the process of mixing a cement-water slurry and pumping it down through steel casing to critical points located in the annulus around the casing, in the open hole below, or in fractured formations.
Cementing a well protects possible production zones behind the casing wall against salt water flow and protects the casing against corrosion from subsurface mineral waters and electrolysis from outside. Cementing also eliminates the danger of fresh drinking water and recreational water supply strata from being contaminated by oil or salt water flow through the borehole from formations containing these substances. It further prevents oil well blowouts and fires caused by high pressure gas zones behind the casing and prevents collapse of the casing from high external pressures which can build up underground.
A cementing operation for protection against the above-described downhole condition is called primary cementing. Secondary cementing includes the cementing processes used in a well during its productive life, such as remedial cementing and repairs to existing cemented areas. The present invention is generally useful in both primary and secondary or remedial cementing. In the early days of oilfield production, when wells were all relatively shallow, cementing was accomplished by flowing the cement slurry down the casing and back up the outside of the casing in the annulus between the casing and the borehole wall.
As wells were drilled deeper and deeper to locate petroleum reservoirs, it became difficult to successfully cement the entire well from the bottom of the casing, and, therefore, multiple stage cementing was developed to allow the annulus to be cemented in separate stages, beginning at the bottom of the well and working upwardly.
Multiple stage cementing is achieved by placing cementing tools, which are primary valved ports, in the casing or between joints of casing at one or more locations in the borehole; flowing cement through the bottom of the casing, up the annulus to the lowest cementing tool in the well; closing off the bottom and opening the cementing tool; and then flowing cement through the cementing tool up the annulus to the upper stage, and repeating this process until all of the stage of cementing are completed.
Some prior art cementing tools used for multi-stage cementing have two internal sleeves, both of which are shear-pinned initially in an upper position, closing the cementing ports in the tool. To open the cementing ports, a plug is flowed down the casing and seated on the lower valve. Fluid pressure is then increased in the casing until sufficient force is developed on the plug and sleeve to shear the shear pins and move the lower sleeve to the position uncovering the cementing ports. Cement is then flowed down the casing and out the ports into the annulus. When the predetermined desired amount of cement is flowed into the annulus, another plug is placed in the casing behind the cement and flowed down the casing to seat on the upper valve. The pressure is increased on the second plug until the shear pins holding it are severed and the upper sleeve is moved down to close the cementing ports. One such cementing tool of this type is disclosed in Baker U.S. Pat. No. 3,768,556, assigned to the assignee of the present invention.
One improvement on the Baker '556 device is found in Jessup et al. U.S. Pat. No. 4,246,968, and also assigned to the assignee of the present invention. The Jessup et al. '968 patent discloses a device similar to that of the Baker '556 patent, except it has added a protective sleeve which covers some of the internal areas of the tool which are otherwise exposed when the internal sleeve is moved downwardly to close the port. This protective sleeve prevents other tools, which may be later run through the cementing tool, from hanging up on the inner bore of the cementing tool.
Another approach which has been utilized for cementing tools is to locate the closure sleeve outside the housing of the tool. One such line of tools is distributed by the Bakerline Division of Baker Oil Tools, Inc., known as the Bakerline Model "J" and Model "G" stage cementing collars. These closure sleeves have a differential area defined thereon and are hydraulically actuated in response to internal casing pressure which is communicated with the sleeves by movement of an internal operating sleeve to uncover a fluid pressure communication port.
An external sleeve cementing tool which uses a mechanical inner locking means between an inner operating sleeve and an outer closure sleeve is disclosed in Giroux et al. U.S. Pat. No. 5,038,862, assigned to the assignee of the present invention. This external sleeve cementing tool is particularly useful in completing stage cementing of slim hole oil and gas wells. Slim hole completions involve using casing inside relative small hole sizes to reduce the cost of drilling the well. In other words, the well annulus between the borehole and the casing is relatively small.
There are cementing applications which necessitate the sealing off of the annulus between the casing string and the wall of the borehole and one or more positions along the length of the casing string. An example of such an application is when it is desired to achieve cementing between a high pressure gas zone and a lost circulation zone penetrated by the borehole. Another application is when it is desired to achieve cementing above a lost circulation zone penetrated by the borehole. A third application occurs when the formation pressure of an intermediate zone penetrated by the borehole is greater than the hydrostatic head of the cement to be placed in the annulus thereabove. Still another application occurs when a second stage of cement is to be placed at a distant point up the hole from the top of the first stage of cement, and a packer is required to further support the cement column in the annulus. A further example of an application for employment of the cementing packer occurs when it is desired to achieve full hole cementing of slotted or perforated liners.
An example of such an inflation packer for cementing is the multi-stage inflatable packer disclosed in Baker U.S. Pat. No. 3,948,322, assigned to the assignee of the present invention. In this device, an open plug is dropped into the casing string and pumped down to actuate an opening sleeve to allow inflation of the packer element. A back check valve prevents the packer from deflating. After the packer is inflated, additional pressure is applied which moves an annular valve member to open a port in the well annulus above the inflated packer element. In a later version of this apparatus, a thin walled secondary opening sleeve is sheared to open this port.
The secondary opening sleeve, being essentially a thin walled mandrel, is difficult to manufacture. Further, when the tool is positioned in the wellbore, there may be some bending of the tool which can cause the annular valve member or secondary opening sleeve to bind and not open as desired. This problem is addressed in Stepp et al. U.S. Pat. No. 5,109,925, also assigned to the assignee of the present invention, in which the annular valve member or secondary opening sleeve is replaced by a secondary rupture disc which is designed to burst or rupture at the predetermined pressure.
A stage cementer and inflation packer which combines the advantages of the external sleeve cementing tool of Giroux et al. '862 with the inflation packer of Stepp et al. '925 is found in Streich et al. U.S. Pat. No. 5,314,015, another patent assigned to the assignee of the present invention. Thus, Streich et al. provides an apparatus which is well adapted for use in slim hole completions and those applications which necessitate the sealing off of the annulus between the casing string and the borehole, while eliminating the binding problems which can result due to slight bending of the tool. The Streich et al. apparatus has the disadvantage of being relatively expensive to manufacture due in part to the need to attach a long packer to the cementer by means of a specially designed coupling. The present invention solves this problem by providing an external cementer configuration with a lengthened mandrel such that the packer element can be assembled directly onto the mandrel. The top end of the packer element overlaps with a closing sleeve to allow a path for fluid to enter the packer from the cementing portion and thereby inflate the packer element. This design allows for a reduced number of parts, simpler assembly, and reduced manufacturing costs relative to the previous external sleeve designs. As a result, the present invention results in an apparatus which is much shorter than the device of Streich et al. and therefore is less expensive to package, transport and handle.
Another possible disadvantage of the previous device of Streich et al. is that the seat retainer therein may not hold its position because the lock ring used to hold the seat retainer in place may slide out of its groove. This is generally undesirable, and the present invention incorporates an improvement in the seat retainer that prevents the lock ring from slipping out of its groove. This has the additional advantage that high pressures may be applied on top of the cementer closing plugs in order to pressure test the casing without movement of the seat retainer which was not possible with the previous design.